This invention concerns an embossed lightweight nonwoven fibrous sheet product having a multiplicity of small transparent embossed areas substantially uniformly distributed across its surface and a method for preparing the same. The production of nonwoven sheets from continuous networks of film-fibril elements is described in U.S. Pat. No. 3,169,899 wherein a solution of polymer is flash-spun at a temperature above the boiling point of the solvent and at high pressure into a low pressure region, whereupon a three-dimensional network of film-fibrils forms at the spinneret. The continuous network is spread laterally by means of a baffle and is then collected in multidirectional, overlapping, and intersecting arrangement on a moving belt. The sheet may be consolidated by passing it through the nip of a pair of cold rolls.
Although such nonwoven sheet products have many uses directly as produced, a more abrasion resistant and delamination resistant product is desirable for certain end-uses. U.S. Pat. No. 3,478,141 describes a thermal point-embossing technique useful for bonding such sheets of film-fibril elements. The embossed regions of these sheets, referred to as point bonds, constitute numerous small areas where the film-fibril elements have been pressure-compacted and partially fused together, thereby decreasing the light scattering ability and increasing the light transmission for these bonded areas, which are accordingly also referred to as "translucent windows". On subsequently being subjected to a suitable mechanical softening treatment, the film-fibril elements in the regions between point bonds are "fluffed up" on a microscopic scale and thereby given more mobility, thus resulting in a soft drapable nonwoven sheet retaining good delamination and abrasion resistance by virtue of the residual point bonds. Such sheets are useful in disposable garments, as drapes and curtains, as protective packaging, etc.
In many such applications, it is desirable to provide a lighter weight sheet product, and this is in fact practical down to sheet basis weights of around 1.3 oz/yd.sup.2 using the thermal point embossing technique. When such sheets are prepared in still lighter weights, they present variations in light transmission and reflection occasioned by local nonuniformity in sheet basis weight, which precludes their full acceptance in such applications even though their tensile properties are in fact fully adequate. This is due to the fact that low basis weight sheets offer less opportunity for averaging out nonuniformities since fewer layers of film-fibril network are present.